In recent years, CMOS sensors as solid-state image sensing elements that use MOS transistors have been extensively developed. FIG. 1 is a schematic diagram of a CMOS sensor.
Reference numeral 1 denotes pixel circuits each of which has a photodiode used to convert light into an electrical signal, and a transistor. Such pixels line up in the horizontal and vertical directions to form a two-dimensional (2D) array. Reference numeral 2 denotes vertical output lines used to output signals from the pixels; 3, signal lines used to transfer voltages to transistors in the pixels; 4, a vertical scan circuit for outputting pulses in turn to the signal lines 3 in the vertical direction; 5, load transistors each of which forms a source-follower circuit with the transistor in each pixel; 6, a read circuit for reading noise signals and photoelectric conversion signals from the pixels; and 7, a differential amplifier for executing a differential process between the optical signal and noise signal.
FIG. 2 is a detailed equivalent circuit diagram of the pixel circuit 1 explained using FIG. 1. Reference numeral 21 denotes a photodiode which serves as a photoelectric conversion unit for converting light into an electrical signal; 22, an amplification transistor which receives a signal generated by the photodiode at its gate electrode, and amplifies and outputs that signal from its source electrode; 23, a transfer transistor which transfers the signal from the photodiode to the amplification transistor 22; 24, a reset transistor which supplies a reset potential to the gate electrode side (floating diffusion 26) of the amplification transistor; and 25, a selection transistor which selectively output a signal in the pixel onto the vertical output line.
The operation of the aforementioned CMOS sensor will be briefly explained below. The transfer transistor 23 and reset transistor 24 are turned on to reset the photodiode 21 and floating diffusion 26. After that, the transfer transistor 23 and reset transistor 24 are turned off, and the photodiode 21 starts accumulation of a photocharge.
During accumulation of the photocharge by the photodiode 21, the selection transistor 25 is turned on to read out a noise signal (a potential corresponding to that which resets the floating diffusion). After that, the transfer transistor 23 is turned on to transfer the photocharge to the floating diffusion, and a potential (photoelectric conversion signal) corresponding to that of the floating diffusion is read out.
The photoelectric conversion signal and noise signal are input to the differential amplifier 7 via the read circuit 6 to remove noise components contained in the input photoelectric conversion signal.
When the transfer transistor 26 is turned on, since it operates in a triode region (linear region), the source and drain sides are coupled via a channel, as shown in FIG. 3. FIG. 4 is a potential chart immediately after the gate of the transfer transistor 23 is enabled.
That is, as shown in FIG. 5, the photodiode 21 (electrostatic capacitance C21) and floating diffusion 26 (electrostatic capacitance C26) are capacitively coupled. Hence, if Vo represents a voltage generated by the photodiode 21, the voltage of the floating diffusion 26 is given by {C26/(C21+C26)}Vo due to capacitive division, and the voltage decreases, resulting in a sensitivity drop.
Upon resetting the photodiode, when the transfer transistor 23 operates in a triode region, the reset potential of the photodiode is influenced by variations of a threshold value of the transfer transistor 23, thus producing fixed pattern noise.